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rudists
Rudists are a group of box-, tube-, or ring-shaped marine heterodont bivalves that arose during the Jurassic and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean.
== Shell description ==
The Jurassic forms were elongate, with both valves being similarly shaped, often pipe or stake-shaped, while the reef-building forms of the Cretaceous had one valve that became a flat lid, with the other valve becoming an inverted spike-like cone. The size of these conical forms ranged widely from just a few centimeters to well over a meter in length.
Their "classic" morphology consisted of a lower, roughly conical valve that was attached to the seafloor or to neighboring rudists, and a smaller upper valve that served as a kind of lid for the organism. The small upper valve could take a variety of interesting forms, including: a simple flat lid, a low cone, a spiral, and even a star-shaped form (Johnson, 2002).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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